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Copyright of Nonferrous Metals (Extractive Metallurgy) is the property of Beijing Research Institute of Mining & Metallurgy Technology Group and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A study on the siderite tailings for treatment of arsenic sulfide slag with arsenic waste acid by utilizing siderite tailings as an iron source and arsenic-containing acidic waste acid as a neutralizing agent. The results showed that the removal efficiency of As reached up 99. 59%, and the TCLP leached concentration of arsenic was only 0.10 mg/L with arsenic waste acid dosage V(ASS)/V(WSA) of 1: 2, Fe/As molar ratio of 3, and hydrothermal temperature of 140 °C for 14 h・. [ABSTRACT FROM AUTHOR]

Arsenic is a common pollutant and impurity present in complex gold ores. In the pressure oxidation (POX) of refractory gold-bearing sulfides, a major environmental challenge is the treatment of the hazardous waste released from arsenic-bearing minerals during processing. While the bulk removal of arsenic from solution can occur during POX, the formation of stable arsenates relies on the operating conditions during POX and the subsequent curing stage. Herein, response surface methodology (RSM) and central composite design have been investigated as viable approaches for optimizing arsenic fixation during the curing of the POX product of arsenopyrite. Curing time (0–24 h) and temperature (60–120°C) were examined as the model variables for RSM optimization, and the performance was assessed via arsenic and iron precipitation, along with the change in free acid and sulfate concentrations. Experimental validation of the optimized model conditions demonstrated good agreement with the simulated outputs and provided a 10% increase in arsenic removal over the best model input. The formation of basic ferric arsenate sulfate and scorodite under these conditions was supported by RSM and confirmed via characterization. In the investigated system, the maximum arsenic removal occurs at a critical threshold temperature of 107°C, over which the scorodite formation decreases with temperature. Thermodynamic modeling revealed the preferable formation of soluble FeHAsO4+ complexes over scorodite above this threshold temperature, decreasing arsenic fixation at higher temperatures. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) has been used with CRYSTAL14 program to optimize hydrogen atom positions in the crystal structure of parascorodite Fe(AsO4)⋅2H2O. It has been shown that taking into account hydrogen bonding, the structure forms six-membered asymmetrical cycles –Fe1–O1–H1...O3–As–O2– and eight-membered cycles –Fe1–O1–H2...O2–Fe1–O2...H2–O1– with lateral topological symmetry. Calculation of the structural complexity parameters for scorodite and parascorodite and their comparison with thermodynamic characteristics shows that the Fe(AsO4)⋅2H2O polymorph modifications correspond well to Goldsmith's rule that states that metastable transitional polymorphs are structurally simpler than thermodynamically stable phases. Scorodite is a stable phase, whereas parascorodite is metastable, which agrees well with the conclusions made in the previous works. Crystallization of parascorodite under natural conditions of oxidation zones of ore-mineral deposits may occur due to the high speed of its nucleation as a metastable phase, i.e., due to the kinetic stabilization of its structure. [ABSTRACT FROM AUTHOR]

Arsenic-alkali slag (AAS) is one of the most hazardous solid wastes generated by the antimony smelting process, posing a significant environmental risk. However, the economic method for effective As immobilization in AAS is still lacking. In this study, we proposed an economical and novel method for the removal of As from AAS using siderite (FeCO3). After the sodium carbonate is recovered from the AAS leaching liquor, the process continues. Aqueous As(V) is immobilized as scorodite via continuous dissolution–oxidation of siderite (FeCO3) by air injection. The residual As(V) is immobilized via Fe-As coprecipitation by lime neutralization. The dissolution–oxidation of siderite produces Fe(III), ensuring continuous Fe(III)-As(V) coprecipitation and scorodite crystallization. The results showed that the removal efficiency of As from AAS reached 99.98%. The mixture of scorodite and Fe-As coprecipitation demonstrated good As stability in the Toxicity Characteristic Leaching Procedure (2.6 mg/L) and at pH 4 (1.84 mg/L), pH 6 (2.13 mg/L), and pH 8 (3.20 mg/L) in a 50-d long-term stability test. Combined results from chemical analysis, X-ray diffraction, and infrared spectroscopy revealed that the existence of high ionic strength Na+ and Cl– were the critical factors in the dissolution of siderite and the crystallization of scorodite. This study provides an effective treatment method for the removal and fixation of arsenic in metallurgical AAS. [ABSTRACT FROM AUTHOR]

In order to deposit arsenic residues from copper production in a stable way, the trivalent arsenic must first be xidized to arsenic(V). A well-known but quite expensive method for this is oxidation with hydrogen peroxide. In order to enable the oxidation of arsenic on a large scale in the future, a potentially cheaper method has to be found, which offers the possibility of oxidizing extremely high arsenic concentrations. As a novel alternative, electrochemical oxidation using a boron-doped diamond electrode is investigated. Based on previous work, this paper concentrates on the presence of interfering ions during oxidation. Furthermore, it is shown that the electrochemically xidized arsenic(V) can be precipitated as scorodite. Finally, an economic analysis shows the potential financial benefit of oxidation via BDD electrodes compared to hydrogen peroxide. [ABSTRACT FROM AUTHOR]

Scorodite (FeAsO 4 ·H 2 O) is a common arsenic-bearing (As-bearing) iron mineral in near-surface environments that could immobilize or store As in a bound state. In flooded soils, microbe induced Fe(III) or As(V) reduction can increase the mobility and bioavailability of As. Additionally, humic substances can act as electron shuttles to promote this process. The dynamics of As release and diversity of putative As(V)-reducing bacteria during scorodite reduction have yet to be investigated in detail in flooded soils. Here, the microbial reductive dissolution of scorodite was conducted in an flooded soil in the presence of anthraquinone-2,6-disulfonate (AQDS). Anaeromyxobacter, Dechloromonas, Geothrix, Geobacter, Ideonella , and Zoogloe a were found to be the dominant indigenous bacteria during Fe(III) and As(V) reduction. AQDS increased the relative abundance of dominant species, but did not change the diversity and microbial community of the systems with scorodite. Among these bacteria, Geobacter exhibited the greatest increase and was the dominant Fe(III)- and As(V)-reducing bacteria during the incubation with AQDS and scorodite. AQDS promoted both Fe(III) and As(V) reduction, and over 80% of released As(V) was microbially transformed to As(III). The increases in the abundance of arrA gene and putative arrA sequences of Geobacter were higher with AQDS than without AQDS. As a result, the addition of AQDS promoted microbial Fe(III) and As(V) release and reduction from As-bearing iron minerals into the environment. These results contribute to exploration of the transformation of As from As-bearing iron minerals under anaerobic conditions, thus providing insights into the bioremediation of As-contaminated soil. [ABSTRACT FROM AUTHOR]

Copyright of Mining & Metallurgy (10057854) is the property of Beijing Research Institute of Mining & Metallurgy Technology Group and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Arsenic, one of the most toxic elements, is a byproduct generated from metallurgical processes of arsenic-containing ores. Arsenic can be formed into an arsenic trioxide as a byproduct during the decopperization in electro-refining process when processing high arsenic-bearing copper minerals. However, arsenic trioxide has a very high arsenic mobility, so it requires to be re-dissolved and immobilized into scorodite (FeAsO4▪2H2O). In this study, methanesulfonic acid (MSA) was tested for both arsenic trioxide leaching and scorodite precipitation. Arsenic trioxide leaching tests showed the arsenic extraction of 80% at 21°C in MSA with H2O2 as an oxidant within 6 hours. Meanwhile, arsenic precipitation tests were conducted to achieve above 95% arsenic removal within 6 hours by the formation of scorodite at an initial pH of 0.5 and temperature of 80°C. In addition, coarser scorodite particles were formed in higher temperature and lower solution pH. The optimum condition for scorodite crystallization in MSA medium can be suggested to be at 80°C with the initial pH of 0.5. [ABSTRACT FROM AUTHOR]

In situ treatment of acidic arsenic-containing wastewater from the non-ferrous metal smelting industry has been a great challenge for cleaner production in smelters. Scorodite and iron arsenate have been proved to be good arsenic-fixing minerals; thus, we used lead slag as an iron source to remove arsenic from wastewater by forming iron arsenate and scorodite. As the main contaminant in wastewater, As(III) was oxidized to As(V) by H2O2, which was further mineralized to low-crystalline iron arsenate by Fe(III) and Fe(II) released by lead slag (in situ generated). The calcium ions released from the dissolved lead slag combined with sulfate to form well-crystallized gypsum, which co-precipitated with iron arsenate and provided attachment sites for iron arsenate. In addition, a silicate colloid was generated from dissolved silicate minerals wrapped around the As-bearing precipitate particles, which reduced the arsenic-leaching toxicity. A 99.95% removal efficiency of arsenic with initial concentration of 6500 mg/L was reached when the solid–liquid ratio was 1:10 and after 12 h of reaction at room temperature. Moreover, the leaching toxicity of As-bearing precipitate was 3.36 mg/L (As) and 2.93 mg/L (Pb), lower than the leaching threshold (5 mg/L). This work can promote the joint treatment of slag and wastewater in smelters, which is conducive to the long-term development of resource utilization and clean production. [ABSTRACT FROM AUTHOR]

Sodium arsenate crystals are a hazardous waste produced from the utilization of arsenic–alkali residue in antimony metallurgy that pose a serious threat to the environment. In this article, to eradicate the damage caused by sodium arsenate and enhance the sustainable development of nonferrous metallurgy, a novel process was proposed to immobilize arsenic as flaky scorodite using Fe(OH) 3 as an iron source. Various methods, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and inductively coupled plasma atomic emission spectrometry (ICP AES), were applied to characterize these synthesized products. The whole process is divided into three parts. In the thermodynamic analysis, the stable area of scorodite in the Pourbaix diagram significantly expands at high ionic activity and temperature, which favors the decrease in △ r G for scorodite synthesis, and Fe–As coprecipitation is feasible in solution at 0.18 ≤pH≤ 5.30. In the scorodite synthesis, 99.88% of the arsenic was converted to a flaky scorodite with a size of 2–5 µm by optimizing the parameters. In the removal of arsenic, residual arsenic and iron were completely coprecipitated into an iron salt by dropping NaOH solution to a final pH of 5.0. The final products include scorodite and sodium sulfate in this process. Overall, this new approach successfully eliminates the potential arsenic pollution from the utilization of arsenic–alkali residue in antimony metallurgy and can potentially be applied to dispose of other high arsenic-bearing wastes. [ABSTRACT FROM AUTHOR]

corodite(FeAsO4 • 2H2O) precipitation method is used for the non-ferrous metallurgy arseniccontaining wastewater, a study on the effect of ionic strength on the removal efficiency of arsenic from wastewater and crystallization of scorodite, and the reaction mechanism is explained. The experiments have shown that ionic strength only affects the removal efficiency of As, arsenic removal efficiency of - 98% even at higher ionic strength. XRD, SEM, IR and Raman show that ionic strength only decrease the rate of crystallization of scorodite, and does not affect the final crystallization of scorodite. The result show that the scorodite precipitation method has a good arsenic fixation ability for high ionic strength arsenic-containing wastewater. [ABSTRACT FROM AUTHOR]

The incorporation of sulfate ( S O 4 2 − ) into the scorodite (FeAsO4·2H2O) lattice is an important mechanism during arsenic (As) fixation in natural and engineered settings. However, spectroscopic evidence of S O 4 2 − speciation and local structure in scorodite lattice is still lacking. In this study, X‑ray difraction (XRD), Fourier transform infrared spectroscopy (FTIR), sulfur K-edge X‑ray absorption near edge structure (XANES), and extended X‑ray absorption fine structure (EXAFS) spectroscopic analyses in combination with density functional theory (DFT) calculations were used to determine the local coordination environment of S O 4 2 − in the naturally and hydrothermally synthesized scorodite. The S O 4 2 − retention in natural scorodite and the effect of pH value and initial Na+ concentration on the incorporation of S O 4 2 − in synthetic scorodite were investigated. The results showed that trace amounts of S O 4 2 − were incorporated in natural scorodite samples. Scanning electron microscopy (SEM) results revealed that S O 4 2 − was homogeneously distributed inside the natural and synthetic scorodite particles, and its content in the synthetic scorodite increased slightly with the initial Na+ concentration at pH of 1.2 and 1.8. The FTIR features and XANES results indicated that the coordination number (CN) of FeO6 octahedra around S O 4 2 − in scorodite lattice is four. The DFT calculation optimized interatomic distances of S-O were 1.45, 1.46, 1.48, and 1.48 Å with an average of ~1.47 Å, and the interatomic distances of S-Fe were 3.29, 3.29, 3.33, and 3.41 Å with an average of ~3.33 Å. EXAFS analysis gave an average S-O bond length of 1.47(1) and S-Fe bond length of 3.33(1) Å with a CNS-Fe = 4 for S O 4 2 − in the scorodite structure, in good agreement with the DFT optimized structure. The results conclusively showed that S O 4 2 − in the scorodite lattice may be in the form of a Fe2(SO4)3-like local structure. The present study is significant for understanding the formation mechanism of scorodite in natural environments and hydrometallurgical unit operations for waste sulfuric acid treatment. [ABSTRACT FROM AUTHOR]

Among the mechanisms controlling elevated arsenic (As) speciation in groundwater, dissolution-precipitation of As-bearing solids, possibly as colloids, has not been systematically evaluated even though reported groundwater saturation states often indicate super- or near-saturation with respect to multiple solids. In this contribution, a detailed geochemical analysis was performed on well-constrained groundwater quality data collected through (a) sustained sampling (n = 84) over 2.5 y at a newly-identified site in the middle Gangetic plain of India; and (b) metadata analysis on studies conducted worldwide (n = 414). Groundwater saturation indices, speciation (E H -pH), and mineral solubilities (logC-pH) were calculated, consistent with a carefully-selected and updated thermodynamic database. Results suggest that under oxidizing conditions, secondary precipitation of solids similar to scorodite [FeAsO 4 ·2H 2 O (s) ] and pharmacolite [CaHAsO 4 ·2H 2 O (s) ] influences the As(V) concentrations in groundwater. In addition, groundwater at the investigated site was saturated with calcite [CaCO 3(s) ] and rhodochrosite [MnCO 3(s) ]. Evidence of colloidal forms of As-containing and As-free solids was found from SEM-EDS characterization of solids collected on 0.2 μm filter membranes used to sample groundwater. XPS analysis showed that the relative As(V) and As(III) signatures in these solids were consistent with the prevalence of dissolved As(V) and As(III) in groundwater, independently quantified using IC-ICP-MS. HR-TEM-SAED characterization of these solids indicated the possible presence of poorly crystalline scorodite- and pharmacolite-like phases along with calcite and lepidocrocite [γ–FeOOH (s) ] in a predominantly amorphous matrix. Also, a possible role of Mn in inducing As immobilization in calcite was suggested with the identification of ∼ 1:1 Mn:As atomic ratios in these solids, consistent with significant (p < 0.05) correlation of dissolved total As and total Mn. These findings imply that solubility-driven secondary processes may exert additional controls on the eventual fate and transport of arsenic in mixed-redox state shallow groundwaters, apart from the known primary mobilization mechanisms. To the best of our knowledge, this is the first study that has characterized colloidal arsenic in groundwater and related it to prevailing mechanisms. [ABSTRACT FROM AUTHOR]

Arsenic-containing wastewater, which is mainly generated in nonferrous metal-smelting plants, threatens the natural environment and human health owing to its high toxicity and mobility. A promising arsenic immobilization method to solve this problem is to immobilize arsenic as scorodite. In this study, the method of scorodite synthesis using magnetite as the iron source was investigated, which can be performed using low-cost materials under atmospheric pressure without using an autoclave. The generation and crystallization behaviors of the intermediate of the gel-like precursor of scorodite were investigated by monitoring the concentration of each iron ion in solution and characterizing the precipitate. By clarifying the effect of the solution pH for each reaction step, the mechanism and suitable conditions of this process are discussed. [ABSTRACT FROM AUTHOR]

With the objective of enhancing the stability of scorodite, its encapsulation with hydroxyapatite (Ca5(PO4)3OH) (HAP) and fluorapatite (Ca5(PO4)3F) (FAP) surface coatings, the two most stable of the calcium phosphates, inert to pH and redox potential variations, are presented in this work. The experimental work includes: (1) determination of the metastable zone for HAP and FAP precipitation, (2) the synthesis of crystalline scorodite under atmospheric conditions using hydrothermal scorodite seed and its characterization, (3) the coating of scorodite with hydroxyapatite and fluorapatite with supersaturation-controlled heterogeneous crystallization, and (4) the long-term stability of the encapsulated scorodite solids. Hydroxyapatite and fluorapatite were prepared with homogeneous precipitation from a metastable solution to which reagents were added at a controlled flow rate. Crystalline scorodite was produced with seeding precipitation and encapsulated with a direct apatite (HAP or FAP) deposition that was controlled by adjusting the pH and reagent addition. The stability tests in oxic and anoxic environments over the pH range of 5–9 showed the release of arsenic from the apatite-coated scorodite to be much lower than from naked scorodite, thereby demonstrating that apatite-based encapsulation of hazardous materials is technically feasible and merits further consideration for development into an arsenic stabilizing technology. [ABSTRACT FROM AUTHOR]

In this study, we proposed a moderate oxidation strategy for accelerating the oxidative dissolution of zerovalent iron (ZVI) using sulfite (S(IV)), thereby improving the removal of As(V) and As(III). Results revealed that, in the presence of 2.0 mM S(IV), both As(V) and As(III) were selectively converted into scorodite at pH 0 3.0-7.0, while As(III) oxidation and As(V) immobilization were impressed over pH 0 8.0-10.0. Batch experiments, radical quenching experiments, and electron spin resonance (ESR) measurements demonstrated that ZVI initially boosted S(IV) activation to generate SO 4 •- OH, and protons, and in turn, ZVI was further oxidized more intensely by these radicals than by oxygen. Concurrently, substantial protons derived from S(IV) oxidation neutralized hydroxyls produced by ZVI oxidation, maintaining an acidic environment conducive to the generation of scorodite rather than iron (hydr)oxides. Characterizations of X-ray diffraction (XRD), Raman, attenuated total reflectance-Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM) confirmed that scorodite was formed in situ and then exfoliated from the surface of ZVI, and approximately 75% of ZVI could still be recovered, which contributed to efficient As removal in successive runs and real As-polluted wastewater. The application of S(IV) achieved a balance among ZVI reactivity improvement, As(V)/As(III) removal, and raw material consumption, making it a promising approach for addressing arsenic contamination in wastewater treatment.• OH, and protons, and in turn, ZVI was further oxidized more intensely by these radicals than by oxygen. Concurrently, substantial protons derived from S(IV) oxidation neutralized hydroxyls produced by ZVI oxidation, maintaining an acidic environment conducive to the generation of scorodite rather than iron (hydr)oxides. Characterizations of X-ray diffraction (XRD), Raman, attenuated total reflectance-Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM) confirmed that scorodite was formed in situ and then exfoliated from the surface of ZVI, and approximately 75% of ZVI could still be recovered, which contributed to efficient As removal in successive runs and real As-polluted wastewater. The application of S(IV) achieved a balance among ZVI reactivity improvement, As(V)/As(III) removal, and raw material consumption, making it a promising approach for addressing arsenic contamination in wastewater treatment.

[Display omitted] • Formation of scorodite via As co-precipitation during Fe(II) oxidation. • Scorodite formation possible up to Fe(II):As(V) of 6:1, in contrast with Fe(III). • Effect of cations on As precipitation in the order of Cu >>> Zn > Al > Ca ∼ Mg > Baseline > Na. • Seawater as a reaction medium reduces As precipitation by less than 4 %. • Potential for Fe(III)-As pre-reduction to facilitate scorodite formation. In recent years, one of the more effective developments in the fixation of arsenic from nonferrous waste has been the synthesis of scorodite via the co-precipitation of arsenic with iron during Fe(II) oxidation. This work examines the impact of cation impurities on the process with the aim of widening the application of this approach to incorporate the use of seawater and the treatment of ferrous metallurgy waste. Scorodite formation was promoted by the presence of cations in the order of Cu2+ >> Zn2+ > Al3+ > Ca2+ ∼ Mg2+, with differences in precipitation behaviour, yield and kinetics due to specific cation effects on iron oxidation. A grain size increase was observed with Cu, while the opposite was found with Al, attributed to crystallisation from solution over epitaxial growth. The use of seawater decreased As precipitation by <4 % without affecting grain size, showing the potential to be utilised as reaction medium. While the presence of Na+ hinders As precipitation, the impact appears less pronounced with seawater due to competing effects from other cations. Since scorodite precipitation is slowed down by high Fe(III) in the feed, a pre-reduction approach with Al, Cu and Zn base metals was examined for conversion of Fe(III) in metallurgical waste to Fe(II) prior to co-precipitation with As. While this produced stable scorodite precipitates, the yield and presence of unstable by-products were impacted by the method of reductant addition. [ABSTRACT FROM AUTHOR]

Arsenic is a toxic element, and development of effective methods for As removal and stabilization are necessary. Removal and stabilization of pentavalent As as crystalline scorodite (FeAsO4·2H2O) is a promising method for As treatment of industrial byproducts. When hematite (Fe(III)2O3) powder is added to As(V) solution containing Fe(II) under appropriate conditions, scorodite crystals form from the gel-like precursor. When Fe(II) is not contained in the solution, the formation reaction does not proceed, even if hematite is added. Therefore, it is considered that the Fe(II) in the solution is heavily involved in the formation mechanism. Here, to elucidate the scorodite crystal formation mechanism in the hematite addition method, the origin of the iron in scorodite was investigated through scorodite synthesis experiments with addition of a stable iron isotope (54Fe) to the reaction solution. It was estimated that the Fe(III) constituting the scorodite crystals was mainly (more than about 80 atom%) derived from the Fe(II) in the solution. From this result, scorodite formation from the reaction solution with solid hematite can be considered to occur as follows. The gel-like precursor is composed of Fe(II) from the reaction solution together with Fe(III) from hematite. During conversion of the gel-like precursor to scorodite crystals, Fe(II) in the precursor is oxidized by Fe(III), and it then combines with the arsenate ion to form scorodite. With conversion to scorodite crystals, the Fe(II) generated by this electron exchange (originally from hematite) dissolves in the reaction solution. It is speculated that electron exchange between solution-derived Fe(II) and hematite-derived Fe(III) plays an important role in formation of scorodite crystals in this synthetic method. [ABSTRACT FROM AUTHOR]

The necessity of arsenic (As) removal from metallurgical wastewaters is increasing. Despite its wide recognition as a natural oxidant, the utility of Mn oxide for scorodite production is mostly unknown. In acidic solutions containing both As(III) and Fe2+, simultaneous oxidation of the two progressed by MnO2 and the resultant As(V) and Fe3+ triggered the formation of crystalline scorodite (FeAsO4·2H2O). At 0.5% or 0.25% MnO2, 98% or 91% As was immobilized by day 8. The resultant scorodite was sufficiently stable according to the TCLP test, compared to the regulatory level in US and Chile (5 mg/L): 0.11 « 0.01mg/L at 0.5% MnO2, 0.78 « 0.05mg/L at 0.25% MnO2. For the oxidation of As(III) and Fe2+, 54% (at 0.5% MnO2) or 14% (at 0.25% MnO2) of initially added MnO2 remained undissolved and the rest dissolved in the post As(III) treatment solution. For the Mn recycling purpose, the combination of Mn2+-oxidizing bacteria and biogenic birnessite (as homogeneous seed crystal) was used to recover up to 99% of dissolved Mn2+ as biogenic birnessite ((Na, Ca)0.5(MnIV,MnIII)2O4·1.5H2O), which can be utilized for the oxidation treatment of more dilute As(III) solutions at neutral pH. Although further optimization is necessary, the overall finding in this study indicated that Mn oxide could be utilized as a recyclable oxidant source for different As(III) treatment systems. [ABSTRACT FROM AUTHOR]

Arsenic is a highly toxic and carcinogenic element, which poses a huge threat to millions of people. The toxicity of arsenic links to its solubility. Therefore, the immobilization of arsenic is of great significance. Among a series of arsenic immobilization materials (include arsenate, arsenite and arsenide), scorodite is the most suitable one due to its excellent stability. In this study, we propose a novel method to synthesize scorodite using ferrihydrite as an in situ iron donator. The process occurs via the dissolution of ferrihydrite followed by the coprecipitation of iron and arsenic ions on the surface of ferrihydrite. A series of experiments were carried out to investigate the optimal reaction conditions and applicable scope of initial arsenic concentration for this process. The results indicated that 99.9% of arsenic was removed from aqueous solution and immobilized as stable scorodite at reaction time of 6 h, pH 1.5, Fe/As molar ratio of 1.1 and reaction temperature of 90°C. This process is applicable to the solution with initial arsenic concentration ranging from 1 to 10 g/L, which shows great potential for practical applications.

In situ treatment of acidic arsenic-containing wastewater from the non-ferrous metal smelting industry has been a great challenge for cleaner production in smelters. Scorodite and iron arsenate have been proved to be good arsenic-fixing minerals; thus, we used lead slag as an iron source to remove arsenic from wastewater by forming iron arsenate and scorodite. As the main contaminant in wastewater, As(III) was oxidized to As(V) by H2O2, which was further mineralized to low-crystalline iron arsenate by Fe(III) and Fe(II) released by lead slag (in situ generated). The calcium ions released from the dissolved lead slag combined with sulfate to form well-crystallized gypsum, which co-precipitated with iron arsenate and provided attachment sites for iron arsenate. In addition, a silicate colloid was generated from dissolved silicate minerals wrapped around the As-bearing precipitate particles, which reduced the arsenic-leaching toxicity. A 99.95% removal efficiency of arsenic with initial concentration of 6500 mg/L was reached when the solid–liquid ratio was 1:10 and after 12 h of reaction at room temperature. Moreover, the leaching toxicity of As-bearing precipitate was 3.36 mg/L (As) and 2.93 mg/L (Pb), lower than the leaching threshold (5 mg/L). This work can promote the joint treatment of slag and wastewater in smelters, which is conducive to the long-term development of resource utilization and clean production.

With the objective of enhancing the stability of scorodite, its encapsulation with hydroxyapatite (Ca5(PO4)3OH) (HAP) and fluorapatite (Ca5(PO4)3F) (FAP) surface coatings, the two most stable of the calcium phosphates, inert to pH and redox potential variations, are presented in this work. The experimental work includes: (1) determination of the metastable zone for HAP and FAP precipitation, (2) the synthesis of crystalline scorodite under atmospheric conditions using hydrothermal scorodite seed and its characterization, (3) the coating of scorodite with hydroxyapatite and fluorapatite with supersaturation-controlled heterogeneous crystallization, and (4) the long-term stability of the encapsulated scorodite solids. Hydroxyapatite and fluorapatite were prepared with homogeneous precipitation from a metastable solution to which reagents were added at a controlled flow rate. Crystalline scorodite was produced with seeding precipitation and encapsulated with a direct apatite (HAP or FAP) deposition that was controlled by adjusting the pH and reagent addition. The stability tests in oxic and anoxic environments over the pH range of 5–9 showed the release of arsenic from the apatite-coated scorodite to be much lower than from naked scorodite, thereby demonstrating that apatite-based encapsulation of hazardous materials is technically feasible and merits further consideration for development into an arsenic stabilizing technology.

The Snowy River processing site was used to extract gold for the large historic Blackwater mine, Westland, between 1906 and 1938, and then abandoned. The site is now one of the best-preserved historic gold-processing localities, and access has been enhanced for visitors. The site has some high arsenic (As) concentrations on the surface, with some spots exceeding 40 wt% As. A variety of As-bearing minerals occurs along the former processing stream, from primary sulphides (arsenopyrite, pyrite) at the upstream end to arsenolite (As2O3) and scorodite (FeAsO4.2H2O) formed by the roasting of ore materials at the downstream end. Weathering over the past century of sulphide residues and some of the arsenolite has yielded more scorodite and some Ca arsenates. Minor authigenic As sulphide precipitates have formed on rotting wood. Arsenolite is principally distributed near to the roaster site, and this is the most soluble and potentially toxic mineral on site. Despite the locally abundant arsenolite, only minor As is mobilised into nearby waters. [ABSTRACT FROM AUTHOR]

Hydrous ferric arsenate (HFA) is a common thermodynamically metastable phase in acid mine drainage (AMD). However, little is known regarding the structural forms and transformation mechanism of HFA. We investigated the local atomic structures and the crystallization transformation of HFA at various Fe(III)/As(V) ratios (2, 1, 0.5, 0.33, and 0.25) in acidic solutions (pH 1.2 and 1.8). The results show that the Fe(III)/As(V) in HFA decreases with decreasing initial Fe(III)/As(V) at acidic pHs. The degree of protonation of As(V) in HFA increases with increasing As(V) concentrations. The Fe K -edge extended X-ray absorption fine structure and X-ray absorption near-edge structure results reveal that each FeO 6 is linked to more than two AsO 4 in HFA precipitated at Fe(III)/As(V) < 1. Furthermore, the formation of scorodite (FeAsO 4 ·2H 2 O) is greatly accelerated by decreasing the initial Fe(III)/As(V). The release of As(V) from HFA is observed during its crystallization transformation process to scorodite at Fe(III)/As(V) < 1, which is different from that at Fe(III)/As(V) ≥ 1. Scanning electron microscopy results show that Oswald ripening is responsible for the coarsening of scorodite regardless of the initial Fe(III)/As(V) or pH. Moreover, the formation of crystalline ferric dihydrogen arsenate as an intermediate phase at Fe(III)/As(V) < 1 is responsible for the enhanced transformation rate from HFA to scorodite. This work provides new insights into the local atomic structure of HFA and its crystallization transformation that may occur in AMD and has important implications for arsenic geochemical cycling.

The synthesis of biogenic scorodite combined with the oxidation of As(III) catalysed by granular activated carbon (GAC) was previously demonstrated. However, the colloidal size of the formed scorodite particles is still a bottleneck, as it would hinder the easy separation of the precipitates in a full-scale application. Here, we studied the effect of GAC concentration on biological scorodite precipitation at thermoacidophilic conditions in batch experiments. Higher arsenic removal efficiency and precipitation of larger and more stable scorodite particles were found only in biotic tests and at low catalyst concentration of 4 g L−1. Furthermore, with 4 g L−1 GAC, the Fe and As predominantly precipitated in solution while with 20 g L−1 GAC the Fe and As predominantly precipitated on the GAC. For experiments with 4 and 20 g L−1 of GAC, the average particle size was 66 and 2.6 μm, respectively. This could be explained by the lower saturation level of the solution at the lower GAC level. This study shows for the first time that the oxidative catalytic capacity of GAC can be used to influence crystallization of scorodite.

In this paper, a scheme is proposed for the treatment of arsenic-containing lead slime by the combination of acid pressure oxidation leaching and forming scorodite. On the basis of thermodynamic calculations, the effects of six factors including acid concentration, oxygen partial pressure (pO2), liquid to solid ratio (L/S), agitating speed, leaching time and temperature for the removal of arsenic were studied in an acid pressure oxidation leaching process, then the optimum leaching conditions were established: L/S of 10 mL/g, leaching time of 2.5 h, pO2 of 2.0 MPa, leaching temperature of 170 °C, acid concentration of 100 g/L and stirring speed of 300 r/min. Under the optimal conditions, the leaching rate of arsenic from lead slime reached 99.10% and the arsenic content of the leaching residue was about 0.80%. After a decontamination procedure, the total arsenic concentration in the acid solution obtained from leaching experiments was 37.18 g/L, and the initial pH was 0.50. Finally, as high as 98.5% of arsenic extracted from the lead slime was stabilized in the form of scorodite (FeAsO4·2H2O) by the precipitation process under the following conditions: initial pH value of 1.0, Fe(II)/As molar ratio of 1.3, pO2 of 2.5 MPa, temperature of 160 °C and precipitation time of 2.0 h.

Based on the characteristics of complex composition,high acidity,arsenic and other heavy metal ions in the waste acid produced by the zinc smelting system, zinc waelz slag produced by zinc hydrometallurgy, which containing a large amount of valuable metals was used to treat the waste acid. This paper puts forward the main process of "Leaching waelz slag by waste acid—Arsenic — Iron co-precipitation — Copper-arsenic cementation — Neutralization and hydroxide — Hematite precipitation processWhile realizing the harmless treatment of waste acid,the valuable metal resources in zinc waelz slag are effectively used. The results showed that the leaching rate of copper,iron and zinc was above 90% in the conditions of two stages cross-current leaching,the precipitation rate of arsenic was above 95% and the precipitation residue was crystalline scorodite. The copper precipitating rate from the solution was over 99% and the iron content of hematite precipitation was up to 64. 42% which can be the raw materials of iron and iron red making processes. [ABSTRACT FROM AUTHOR]

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Objective: To enable removal of highly toxic As(III) from acidic waters by inducing indirect microbial As(III) oxidation by Fe-oxidizing bacteria via carbon-assisted redox-coupling between As(III) oxidation and Fe3+ reduction. Results: Carbon-fiber (CF) was shown to function as an electron-mediator to catalyze chemical (abiotic) redox-coupling between As(III) oxidation and Fe3+ reduction. Accordingly, by taking advantage of Fe3+ regeneration by Fe-oxidizing bacteria, it was possible to promote oxidative removal of As(III) as ferric arsenate at moderate temperature. This reaction can be of use under the situation where a high-temperature treatment is not immediately available. Arsenic once concentrated as ferric arsenate on carbon-fibers can be collected to undergo phase-transformation to crystalline scorodite as the next re-solubilization/re-crystallization step at a higher temperature (70 °C). Conclusions: While extremely acidophilic Fe-oxidizing bacteria are widely found in nature, the As-oxidizing counterparts, especially those grown on moderately-thermophilic and mesophilic temperatures, are hardly known. In this regard, the finding of this study could make a possible introduction of the semi-passive, low-temperature As-treatment using readily available Fe-oxidizing bacteria. [ABSTRACT FROM AUTHOR]

Scorodite (FeAsO 4 ·2H 2 O)–water interaction is critical to As distribution and storage in surface environment but is inadequately understood due to ambiguities in the mineral's stability and weathering rate at atmospheric conditions. In the present study we attempted to experimentally determine the thermodynamic and kinetic parameters needed to compute solubility and dissolution rate of scorodite at 25 °C. Experiments were carried out in low pH (1.15) solutions using specially synthesized large scorodite crystals (with Mean Diameter of 27.9 μm). Such experimental conditions ensured the results were not subject to the influence of secondary Fe-bearing phase precipitation and grain size effect. Measured equilibrium concentrations of Fe and As, along with newly published Fe-As complexes association constants, were first used to determine the solubility products K sp and dissolution rates r n at 50–90 °C. The obtained K sp ∼ T and r n ∼ T dependence was then used to derive the Gibbs free energy, enthalpy, entropy, heat capacity, and activation energy for the dissolution reaction. Finally, we extrapolated the measurements to 25 °C and obtained room temperature solubility and dissolution rate, scrutinized the pH effect on dissolution, and analyzed the Δ G ∼ r n relation of the dissolution reaction. Our results show that literature data are likely overestimated scorodite solubility at pH > 4 ∼ 4.5 due to neglecting the effect of ferric iron hydrolysis. Estimated ambient condition dissolution rate is an order magnitude lower than the earlier report, implicating the importance of size effect, but is one to two orders of magnitude higher than that of common rock-forming minerals, cautioning the proposed use of scorodite for As fixation and storage. The determined r n ∼ ΔG relation cannot be fully fitted by the transition state model, particularly at near equilibrium, suggesting dissolution in this study may be controlled by defect-assisted surface reactions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Micro- to nanoscale gold fills the microcracks in arsenopyrite, and is closely associated with Bi minerals. • Gold is polycrystalline and consists of nanocrystallites 20–30 nm in size. • Arsenopyrite is replaced by scorodite. During this sulfide oxidation, second generation of colloidal-like gold was co-precipitated with scorodite. • A possible low-temperature hydrothermal origin of scorodite is indicated. Arsenopyrite and pyrite are important carriers of Au that is present in the form of microscopic and "invisible" gold. Such gold can be expected to be released upon the process of oxidation of these sulfides. In studied gold mineralization at Drenjak, related to the granitoids of the Oligocene-Miocene Kopaonik Ore District, the main arsenopyrite-pyrite-(bismuthinite) sulfide assemblage was formed in the quartz bodies and partly replaced by scorodite (FeAsO 4 ·2H 2 O) and Bi-arsenate. With only up to 5 ppm of gold recorded by LA-ICP-MS, aforementioned sulfide minerals are here practically free from lattice-bound gold. Native gold is present in the mineralization, generally ranging in grain size from 20 µm down to the nanoparticles, as revealed by TEM. It is mainly hosted by arsenopyrite and scorodite. In arsenopyrite, gold is polycrystalline and consists of nanocrystallites 20–30 nm in size. It was deposited after arsenopyrite, healing its cavities and microcracks, often associated with native bismuth and bismuthinite. The abundance of gold and Bi minerals correlates with the presence of scorodite. Two types of gold are present in scorodite and Bi arsenate: i) prevailing relict Au grains retained after arsenopyrite oxidation, ii) colloidal-like gold co-precipitated with the arsenates. Based on our observations, we additionally present and discuss indications of low-temperature hydrothermal origin of scorodite, formerly known only as a weathering product [ABSTRACT FROM AUTHOR]

Scorodite is an important mineral not only for arsenic (As) removal from industrial wastewaters but also in the mobility and final fate of As in waste rocks, contaminated soils and sediments, and mine tailings. Because of the mineral's high As-loading capacity and stability, numerous studies have been done to understand its formation. Unfortunately, most of these studies were limited to elevated temperatures (>70 °C), so the processes involved in scorodite formation under ambient conditions remain unclear. This study provides evidence of the catalytic effects of hematite on the formation of scorodite at 25 °C in a pyrite-rich natural geologic material. Scorodite peaks were detected in the XRD patterns of the leaching residues with and without hematite, but those in the former were stronger and more pronounced than the latter. These results suggest that the formation of scorodite was catalysed by hematite, a generalisation that is further supported by strong characteristic IR absorption bands of scorodite at 819 cm–1 (As–O bending vibration), 785 and 725 cm–1 (As–O stretching vibrations), and 2990 cm−1 (OH-vibration) as well as the distinct XPS binding energies of Fe(III)–As (709.7 eV), As(V)–O (44.8, 44.31 and 43.7 eV), O2− (530.5 eV) and coordinated water (531.3 eV) in scorodite. This phenomenon could be attributed to three possible mechanisms: (1) more rapid precipitation promoted by the "seeding" effect of hematite particles, (2) additional supply of Fe3+ from hematite dissolution under acidic conditions, and (3) enhanced oxidations of Fe2+ to Fe3+ and As(III) to As(V) on the surface of hematite. Image 1 • Scorodite formation was promoted in the presence of hematite. • Hematite particles acted as "seeds" that enhanced scorodite precipitation. • Partial hematite dissolution supplied Fe3+ that enhanced scorodite formation. • Hematite catalysed Fe2+ and As(III) oxidations that promoted scorodite formation. • Hematite could be used to catalyse scorodite formation at room temperature. [ABSTRACT FROM AUTHOR]

In the present study, a process for selective removal of arsenic from Cu–As-containing filter cakes using Na2CO3 leach was examined. When decreasing the weight percentage of arsenic in Cu–As-containing filter cakes, environmental problems can be eliminated while valuable metals will be recycled. The effect of α(Na2CO3) (where α represents excess coefficient), temperature, L/S ratio and leaching time were investigated. the optimal leaching conditions were achieved as follows: α(Na2CO3) = 2.7, L/S ratio 6 : 1 (mL/g), temperature 75°C, leaching time 3 h, where the leaching efficiency were 96.47% for As, 0.41% for Cu with final As and Cu concentrations of 11.72 and 0.02 g/L respectively. Further analysis showed that the Cu content in the leach residue increased to 51.42% with CuS as the major Cu phase. Notably, only 0.92% of As remained in the leach residue. The leaching solution was treated by sodium persulfate oxidation and arsenic precipitation which eliminated As 99%, leaving 0.5 mg/L As in the solution. Therefore, the proposed process is a viable method for the effective separation of Cu and As in Cu–As-containing filter cakes. Broadly, the results of this paper provide a reference for the treatment of arsenic sulfide from nonferrous metallurgy cyclic leaching process. [ABSTRACT FROM AUTHOR]

High arsenic-containing waste acid from the heavy nonferrous metallurgical sector (Cu, Pb, Zn, Ni, Sn, etc.), one of the most dangerous arsenic hazardous wastes with extremely high arsenic concentrations, has presented enormous challenges to the environment and caused severe environmental pollution over the past few decades due to the lack of affordable and environmentally friendly disposal technologies. Here, we report a green process for the self-enhanced and efficient removal of arsenic from waste acid using magnetite as an in situ iron donator. Firstly, the room-temperature predissolution of magnetite in waste acid provides initial iron ions as a starting precipitator of arsenic, simultaneously providing a suitable pH range and an active surface that are ready for the nucleation and growth of scorodite. Afterwards, arsenic is precipitated in form the of scorodite, which is driven by a mutually improved cycle composed of arsenic precipitation and magnetite dissolution on the surface of magnetite particles. This cycle creates a low supersaturation of iron and constant pH in the waste acid, ensuring the continuous precipitation of arsenic as well-crystallized and environmentally stable scorodite by using magnetite as an in situ iron donator via the reaction of 2Fe 3 O 4 + 6H 3 AsO 4 + H 2 O 2 = 6FeAsO 4 + 10H 2 O. Under optimal conditions, including a 6-h room-temperature predissolution, a 12-h atmospheric reaction at 90 °C and a pH of 2.0 with a magnetite dosage at the Fe 3 O 4 /As molar ratio (the molar ratio of Fe 3 O 4 in magnetite to As in waste acid) of 1.33, 99.90% of arsenic was successively removed from waste acid with an initial arsenic concentration of 10300 mg/L. In combination with the good adaptability of this process, the performed case study and prospective process show the successful removal of arsenic from waste acid as well as great potential for large-scale applications. Image 102 • Arsenic removal from waste acid using magnetite as in situ iron source. • Mutually-improved relationship of arsenic precipitation and magnetite dissolution. • Prospective process for the effective treatment of arsenic-containing waste acid. [ABSTRACT FROM AUTHOR]

For the metallurgical industries, the safe disposal of arsenic-bearing waste is animportant issue. In this study, a two-stage arsenic precipitation process was proposed, which consists of formation of scorodite at lower pH followed by precipitation of arsenical ferrihydrite at higher pH. The experimental results show that arsenic can be removed effectively from a solution with arsenic concentration of 5 g/L as precipitate of crystalline scorodite through oxidizing ferrous ions with air stream at pH of 2at first. Then, the pH value was raised to 3 and 4, and almost all the residual arsenic was removed from the solution due to adsorption onto amorphous ferrihydrite at the molar ratio of Fe/As between 2 and 4. The United States Environmental Protection Agency's Toxicity Characteristic Leaching Procedure tests demonstrate that the complex precipitates generated under these conditions (Fe/As ratio of 4, final pH between 3 and 4) were very stable and no leached arsenic can be detected using acetic acid buffer solution (pH 4.93 ± 0.05) in tests of 72 h. Experimental results suggest that a high Fe/As ratio was necessary, so that the excess iron promotes the formation a product of good stability, arsenical ferrihydrite (AsFH: AsO 4 3−·Fe(OH)(H 2 O) 1+x) under a relatively high pH in acidic solution. Unlabelled Image • Arsenic was removed almost completely in a two-stage arsenic precipitate process. • The arsenate adsorbed by ferrihydrite can be transformed into scorodite. • The excess iron can form an arsenic deficient ferrihydrite which could restrain the leaching of arsenic. • The formed complex precipitates were stable in the TCLP test. [ABSTRACT FROM AUTHOR]

The stability of scorodite and scorodite with crystalline polyferric sulfate (PFS) added was investigated using the Toxicity Characteristic Leaching Procedure (TCLP) and leaching tests using NaOH solutions. The leaching behaviors of iron and arsenic were analyzed using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The results indicated that the addition of PFS reduced the concentration of arsenic dissolved into the leaching solution significantly from 10 to 30 mg/L to below 0.01 mg/L through its adsorption and effect on the equilibrium of scorodite dissolution. The PFS addition facilitated the leaching of iron from the scorodite to precipitate as two-line ferrihydrite that coated the surface of scorodite particles. This potentially slowed down the diffusion of arsenic from the scorodite to the leaching solution. The thermodynamic calculations for the scorodite dissolution process showed that the formation/addition of ferrihydrite was effective in controlling the equilibrium arsenic concentration. With consideration of the ferrihydrite formation in the calculations, the scorodite was shown to be stable between pH 2.0–8.8, which was a wider range than the scorodite stability range of pH 2.0–5.8. • The stability of scorodite under circumneutral and alkaline conditions was improved by the addition of crystalline PFS. • The leaching of arsenic from scorodite was controlled by the addition of PFS via adsorption and changing equilibrium. • The model diagram for the leaching process of scorodite with PFS added was drawn. [ABSTRACT FROM AUTHOR]

Abstract A novel method has been proposed to prepare an arsenate (As(V))-containing solution for scorodite formation using an arsenic (As) sulphide residue obtained from a Pb refinery. Preparation of the As(V) solution from the As sulphide residue included As oxidation leaching by alternating addition of a highly concentrated sodium hydroxide (c-NaOH 600 g/L) solution and hydrogen peroxide (H 2 O 2) at 70 °C. This method gave an As leaching efficiency of up to 99% (total % recovery of As up to 90%), while at the same time, other metals could be recovered effectively (e.g., 95% Pb, 97% Cd, 90% Zn, and 96% Cu). Further oxidation and removal of the remaining arsenite (As(III)) in the leached filtrate by H 2 O 2 indicated that the oxidation efficiency of oxidizing As(III) to As(V) was up to 99%. The As(V) solution produced by this process was suitable for As(V) fixation as scorodite. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to characterize the starting materials and output solids. The results showed that the major As phase in the As-containing compound of the initial As sulphide residue was As sulphide. Highly pure elemental sulphur (total % recovery of S up to 30%) was generated during the S(-II) elimination process, and the scorodite from the As(V) fixation process was highly crystalline. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray absorption near edge structure spectroscopy (XANES) were employed to characterize the intermediate filtrate of the process. The results showed that the intermediate filtrate was a multiphase mixture containing As(III), arsenate, sulphate, thio-arsenate, thio-sulphate and polysulphides (S n 2−). The produced scorodite showed good As stability in the TCLP and at pH 3 (0.47 mg/L), pH 5 (0.26 mg/L), and pH 7 (3.41 mg/L) in a 10-day short-term stability test. Highlights • A complete process of treatment arsenic sulphide residue by transforming into scorodite. • Arsenic was extracted effectively from arsenic sulphide residue without producing arsenic containing hazardous materials. • Experiment results shows potential application value of the process in hydrometallurgy industry. [ABSTRACT FROM AUTHOR]

Arsenic–enriched wastewater (A–EW) is a hypertoxic sewage from the utilization of crude antimony oxides in lead anode slime metallurgy. In traditional methods, the H+ accumulation inhibits the arsenic immobilization during scorodite synthesis. In this study, a novel multivalent iron source comprised of Fe(OH) 3 and FeSO 4 ·7H 2 O was proposed to resolve the adverse effects of pH fluctuation during immobilizing A–EW as scorodite. Various approaches, such as scanning electron microscopy and X–ray photoelectron spectroscopy, were applied to characterize the synthesized scorodite. This work was divided into two parts. In thermodynamics, H n AsO 4 (3−n)- (n = 1, 2, 3) and Fe(OH) n (3−n)+ (n = 0, 1, 2, 3) can feasibly coprecipitate as scorodite according to their △ r G m , T θ ranged from −111.10 kJ mol−1 to −33.53 kJ mol−1. In experimental research, A–EW was immobilized as scorodite by optimizing conditions as initial pH = 2.0, molar ratio of Fe to As = 1.2, molar ratio of Fe(II) to Fe(III) = 4:6, arsenic concentration = 40 g/L, and temperature = 95 °C. The arsenic precipitation ratio is 99.60%, and the micromorphology of synthesized scorodite presents a regular octahedron having size of 5–10 μm. The low leachability of As (0.41 mg/L) in toxicity characteristic leaching procedure (TCLP) confirmed that the prepared scorodite is nonhazardous. The solution pH is stable at 2.0 as the H+ depletion (0.5660 mol) by Fe(OH) 3 dissolution and Fe2+ oxidization balanced with that (0.5657 mol) generated from As(V)–Fe(III) coprecipitation. In general, the A–EW was effectively immobilized by proposed multivalent iron source, and can be potentially applied to safely dispose other industrial effluents, such as high arsenic leachates and arsenic–bearing waste acid from nonferrous metallurgy. Scorodite formation mechanism and protons balance in weak acidic solution using a multivalent iron source. [Display omitted] • H n AsO 4 (3−n)- and Fe(OH) n (3−n)+ can coprecipitate as scorodite from △ r G m , T calculations. • 99.60% of A–EW was immobilized as 5–10 μm octahedral scorodite at a stable pH. • Fe(OH) 3 neutralizes extra H+ from oxidative coprecipitation of Fe(II) and As(V). • Sodium salts were recovered from solution by an evaporative crystallization. [ABSTRACT FROM AUTHOR]

Owing to the chemical stability of scorodite, the synthesis of scorodite for arsenic fixation has gained interest as a treatment method for arsenic-containing wastewater generated at nonferrous metal smelting plants and other facilities. Diverse synthesis processes have been investigated such as hydrothermal, oxidation, and solid iron oxide methods. The solid iron oxide method using hematite as an iron source is particularly promising owing to its many advantages. Though the overall reaction equation for this process does not include Fe2+, the reaction is influenced by Fe2+ in the solution, significantly increasing the reaction rate. In the present study, we examine the mechanism of this reaction electrochemically to elucidate the role of Fe2+. By analyzing the open-circuit potentials and the polarization behaviors of each local reaction and conducting short-circuiting experiments based on the predicted local-cell model, we demonstrate the electrochemical nature of the reaction, which involves the reduction of hematite and the oxidation of Fe2+. In addition, the multistep reaction involved in the formation of scorodite is discussed using potential–pH diagrams based on thermodynamic reference data. [Display omitted] • Acceleration mechanism of Fe2+ on the scorodite synthesis using Fe 2 O 3 was elucidated. • Polarization behavior involved in the scorodite formation was clarified. • Thermodynamic discussion was presented based on E –pH diagrams. [ABSTRACT FROM AUTHOR]